Space Heater with Microprocessor Control

ABSTRACT

A space heater has a microprocessor control that is configured to receive signals from at least the (i) ignition sensor associated with the burner, (ii) the flue pressure sensor, (iii) the secondary header temperature sensor, (iv) the fire box thermistor, and (v) the condensate sump level sensor, and in response thereto, control at least the operation of (a) the inducer, (b) the convection fan, (c) the condensate pump, (d) the burner ignitor, (e) the burner gas valve, and (f) the main gas valve.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional application Ser. No. 60/828,404, filed Oct. 6, 2006, the disclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a high efficiency, condensing gas log space heater with an integrated microprocessor control where combustion gases are exhausted through a three stage heat exchanger system allowing exhaust temperatures to be no more than 125° F.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a space heater;

FIG. 2 is a sectional cross-section taken along the line 2-2 of FIG. 1 in the direction of the arrows shown;

FIG. 3 is a sectional side view taken along the line 3-3 of FIG. 2;

FIG. 4 is a part sectional plan view from above taken along the line 4-4 of FIG. 3;

FIG. 5 is a part sectional plan view from above taken along the line 5-5 of FIG. 3;

FIG. 6 is a part sectional plan view from above taken along the line 6-6 of FIG. 3;

FIG. 7 is a top view of the burner assembly of FIG. 1;

FIG. 8 is a front view of the burner assembly of FIG. 1;

FIG. 9 is a front view of a secondary heater exchanger, a tertiary heater exchanger, a primary header extending between the secondary and tertiary heat exchanger, an inducer fan assembly, and a secondary header extending between the tertiary heater exchanger and the inducer fan assembly;

FIG. 10 is a front view of a turbulator inserted into the secondary heat exchanger of FIG. 8;

FIG. 11 is a front view of a turbulator inserted into the tertiary heat exchanger of FIG. 8;

FIG. 12 is a left side view of the secondary header of FIG. 8;

FIG. 13 is an exploded perspective view of an impeller of the inducer fan assembly of FIG. 8;

FIG. 14 is a cross sectional view of an intake/exhaust pipe used with the space heater;

FIG. 15 is a front view of a condensate pump assembly;

FIG. 16 is a state diagram for a microprocessor control of the space heater;

FIG. 17 is a continuation of the state diagram for the microprocessor control shown in FIG. 16;

FIG. 18 is a state diagram for the microprocessor control;

FIG. 19 is a block diagram of inputs to and outputs from the microprocessor control;

FIG. 20 is a use case diagram for the microprocessor control; and

FIG. 21 is a chart showing diagnostic and fault logic for the microprocessor.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

The space heater disclosed herein comprises a concealed three-stage heat exchanger that captures more than 90 percent of the heat energy from the burner and directs warm air into the room in which the space heater is located. Because so much of the heat is extracted by the three-stage heat exchanger, the exhaust can be vented outdoors in conventional PVC piping, thereby saving money, labor, and space on installation when compared to traditional direct vent systems. Additionally, because so much heat is removed by the three-stage heat exchanger, the space heater stays cool to the touch and may be installed in zero clearance applications against combustible building materials.

Referring to FIG. 1 of the drawings, the space heater 5 is illustrated as seen from inside the room in a building to be heated and there is shown the outer casing 6 of the heater with its large neo-ceramic door glass 7, which may include a screen (not shown) fitted thereon. The heater may include external controls 40 for activation of the space heater and lights 42 indicating its mode of operation. The controls may include a transmitter 44 to allow the space heater to be operated remotely. As explained below, the indicating lights may also be used in a diagnostic/fault application.

Referring to FIG. 2, the heater heats the ceramic logs 9 in a fire box or primary heater exchanger 8 and the products of combustion are drawn through a secondary exchanger 17 and a tertiary exchanger 19 by combustion fan 26 through the depending portion of the secondary header. The space heater is a high efficiency heater and combustion air is drawn into the fire box from outside of the room in which the space heater is located. As will be explained below, the preferably combustion air is drawn from the atmosphere and the combustion products and gases are discharged to atmosphere.

Referring to FIG. 2, a convection fan 30 draws air through a lower front opening 32 of the heater casing 6. The convection air passes under the fire box 8 through an electrical enclosure compartment 50 and into a plenum 52 behind the fire box where the convection air flow over and around the tertiary and secondary heat exchanger 19,17, up and over the top fire box 8, where it returns into the room via a return vent 54 at the top front of the space heater.

As shown in FIGS. 2-6 the fire box or primary heat exchanger 8 is cubically shaped enclosure dimensioned to accommodate the ceramic logs. The fire box is adapted to be fitted within the external casing 6 of the space heater. Heat shielding is provided around the fire box between the casing to the protect the casing. Referring to FIG. 2, in the fire box or primary heat exchanger, ceramic logs 9 are supported on ceramic grate 10 having apertures 11 therein to allow passage of gas flames from a burner plate assembly 12 mounted above an air inlet manifold 13 that extends below and through the rear 14 of the casing. The air inlet manifold 13 communicates with an intake portion 15 and intake/exhaust pipe as discussed below in reference to FIG. 14. A gas control valve 33 is located at the back of the heater and connected to a gas supply and to the burner plate assembly 12. The ceramic grate 10 acts as an insulating medium for the gas and electronic components located in the electronics enclosure 50. The ceramic grate 10 and logs 9 at the same time act as an agent in improving the combustion process in fire box 8 by keeping a high temperature after the gas flames have issued from burners 12 and thereby producing a clean total combustion. The ceramic logs 9 absorb heat from the gas flames whereby thermal energy from logs 9 radiates as infra-red heat through the transparent or semi-transparent door glass 7 at the front of the heater 5.

Referring to FIGS. 7-8, the space heater burner late assembly 12 comprises two burners 60,62 rated for delivering different amounts of heating, for instance, the first burner delivering 9,000 BTUs and the rear burner delivering 19,000 BTUs. The front burner 60 may be operated alone as desired, and when additional heating is required, the second burner 62 may be switched on to maximize heat output. As will described in greater detail below, the space heater is provided with a control system that allows operation of the front burner 60 or the rear burner 62 independently, or both burners together. The space heater may also be provided with a thermostat to control the operation of the burners in accordance with ambient room temperature.

Referring to FIGS. 7-8, the front and rear burners 60,62 are mounted on a burner plate 64 along with a respective hot surface igniter 66,68 and flame detector 70,72 adjacent each burner so that each burner may be operated independently, as discussed previously. The burner plate 64 has openings 74,76 surrounding each burner. Combustion air is directed to air manifold 13 and into the fire box from under the burner plate 64 and through the openings 74,76 around the burner. The openings in the burner plate are dimensioned to maximize the combustion process by allowing the proper amount of combustion air to be introduced into the burner, taking into account the burner capacity and the capacity of the inducer fan 26.

In one embodiment, the inducer motor capacity remains fixed regardless of whether the front, rear, or both front and rear burners are activated in order to simplify construction. Thus, the amount of air introduced to the front burner and rear burner is the same regardless of whether the front, rear, and/or front and rear burners are activated. Thus, the amount of combustion air is dictated by the burner plate openings for each of the respective burners.

In another embodiment, the inducer fan speed is varied in accordance with the burner operation, as will be explained below in greater detail with respect to the inducer motor control. By way of example, for the burner arrangement shown in FIGS. 7-8, the inducer fan may operate at three speeds: a first speed when the smaller rated front burner is operated; a second speed higher than the first speed setting when the rear burner is operated; and a third speed higher than the second speed setting, when both the front and rear burners are operated. In this way, the efficiency of the combustion process can be maximized.

Referring to FIGS. 3-4, a baffle chamber 16 is located at the upper part of the fire box 8 which allows combustion products from the fire box 8 to pass around the edges thereof (FIG. 4) into a secondary heat exchanger 17. The baffle chamber 16 acts to ensure even distribution and balanced extraction of final combustion gases.

Referring to FIGS. 3-4, after exiting the primary heat exchanger 8 through the baffle chamber 16, the combustion gases enter the secondary heat exchanger 17 located in the plenum 52 behind the firebox that communicates with the main convection blower fan 30. The secondary heat exchanger 17 comprises a corrugated flexible stainless steel tube 18, such as a flex parco hose, that relieves thermal expansion stresses between connections of the primary heat exchanger 8 and the tertiary heat exchanger 19. Preferably, the hose is stainless steel although other materials may be used for the hose. Corrosion is generally not a concern in the secondary heat exchanger due to the elevated temperatures at which the secondary heat exchanger operates. A turbulator as shown in FIG. 10, may be provided in the secondary heat exchanger to improve the efficiency of heat transfer. A further discussion of the turbulators follows below.

Referring to FIGS. 3 and 9, once the combustion products exit the secondary heat exchanger 17, they are directed to the primary header 22 that is in communication with the tertiary heat exchanger 19. The primary header 22 is arranged vertically in the compartment behind the firebox with the top of the header receiving the combustion gases from the secondary heat exchanger and the bottom of the header directing the combustion gases into the tertiary heat exchanger. The primary header 22 comprises a header cover 80 attached to a header plate 82 with quick disconnect fasteners 84. The quick disconnect fastener facilitate access to both the secondary heat exchanger and the tertiary heat exchanger by removing the header cover 80 from the header plate 82. The tubes comprising the secondary and tertiary heat exchanger may be secured to their respective header plates.

Referring to FIG. 3 and FIG. 9, the tertiary heat exchanger 19 comprises a plurality of externally finned tubes 23 also located in the plenum 52 behind the firebox and below the secondary heat exchanger. The finned tubes 23 extend generally horizontally but angled slightly downward from the primary header 22 and the secondary header 24. The secondary header 24 is also arranged vertically in the compartment behind the firebox. The finned tubes 23 may be secured to the primary and secondary headers 22,24, by expanding the ends of the tubes 23 in their attachment to the headers to expand both surface sides of the header plates thus creating a very strong crimp fixture which is reliably gas tight with no requirement for welding. The finned tubes are expanded into header plates to avoid welding of finned tubes to primary and secondary header plates to avoid corrosion taking place. When welding is performed on stainless steel, it tends to remove surface protection of the stainless steel, and eventually, due to the presence of condensation, the finned heat exchanger tubes may corrode in areas where they have been welded to the header plates. The finned tubes 23 preferably comprise stainless steel tubes with attached helical corrugated fins 28 (FIG. 9) on the external surfaces thereof to increase the thermal energy transfer to the outer surface of tubes 23. The finned tube 23 may be made from 29-4C stainless steel, as the tertiary heat exchanger may experience some condensation as the combustion products are cooled. The angled arrangement of the finned tubes facilitates draining of condensate from the tubes into the secondary header 24.

Referring to FIGS. 10 and 11, inside the secondary heat exchanger tube 18 and the tertiary heat exchanger tubes 23 are helical aluminium strip or tape 21,29, which acts as a turbulator to the combustion gases travelling through tubes. The turbulators are formed in an arrangement which is helical or twisted. As the combustion gases are drawn through the tubes of the respective heat exchangers and over the turbulators, the flow rate of the gases is slowed and the gases are spiraled outward to the tube wall for more efficient heat transfer.

The turbulators in the secondary and tertiary heat exchanger not only act to increase efficiency, they also act as sacrificial anodes to protect the complete heat exchanger against corrosion, i.e., the secondary heat exchanger, primary header plates, secondary header plates, and the tertiary heat stainless steel finned tubes. With condensation in the system, the turbulators will corrode first before the other materials are attacked. When the turbulators sufficiently corrode, they are no longer efficient in creating a spiral flow of exhaust gases in their respective heat exchanger tubes. Accordingly, after significant corrosion, the temperature through the heat exchangers may increase to a level sufficient to activate the secondary header over temperature limit switch, as discussed further below, which will then shut the gas supply to the heater to protect the heat exchanger and PVC components in the heat exchanger. Thus, the turbulators provide benefits in increasing the efficiency of the heat exchangers and protecting the materials used in the heat exchanger construction to the point where the heat exchanger need not be replaced, but rather the turbulators. The primary and secondary headers are constructed in such a way that they are removable from the secondary and tertiary heat exchangers so that the turbulators can always be replaced and inserted into each tube when corroded.

This is especially critical in the tertiary heat exchanger where condensation is formed. Preferably, the turbulators 29 in the tertiary heat exchanger 19 are made from thin aluminum. By using the turbulators which are removable and act as sacrificial anodes, the finned tubes 23 comprising the tertiary heat exchanger may be made from more inexpensive materials. Thus, the turbulators protect the materials used in the secondary and tertiary heat exchangers to extend the useful life of the heat exchangers and to increase the efficiency of the heat exchangers.

Referring to FIGS. 3, 9 and 12, after exiting the tertiary heat exchanger 19, the combustion gases are directed to the secondary header 24 comprising a header cover 90 and a header plate 92. The header cover 90 may be made from a PVC plastic and attached to the header plate with quick disconnect fasteners 94 to allow access into the tertiary heat exchanger by removing the header cover 90 from the header plate 92. The secondary header 24 has an angled depending portion 100 which communicates with a condensate pump assembly 25 via an outlet port drain 101. The angled arranged ensures that condensate entrained in the combustion gases flows out of the tubes 23 of the tertiary heat exchanger into the secondary header 24 and into the condensate container 31. The secondary header 24 and depending portion 101 may be made from PVC rectangular pipe and include an elbow 102 that provides the necessary angle for draining condensate.

To maximize efficiency of the heat exchanger, the temperature of the combustion gases exiting the tertiary heat exchanger is controlled to be no more than 150° F. degrees. A temperature sensor 104 may be provided on the secondary header to monitor the temperature of the combustion products in the secondary header and provide signaling to shut the space heater off in the event the temperature exceeds a desired amount. Given that the materials comprising the secondary header and the components located downstream of the secondary header are made from PVC or other low temperature materials, continuous monitoring of the temperature of the gases in the secondary header is needed.

A pressure sensor 106 may be provided on the secondary header 24 to assist in the purging cycle of the space heater at start-up. As will be described below, the pressure sensor 106 senses the pressure in the secondary header 24 and compares the signal to atmospheric pressure to determine whether the inducer fan assembly 26 is operating properly and/or the flow path of combustion air and products is unobstructed.

Referring to FIGS. 3, 9, and 13, after exiting the secondary header 24, the combustion gases are drawn to the inducer fan assembly 26. As shown in FIG. 9, the inducer fan assembly may be mounted on the angled depending portion 101 of the secondary header. The inducer fan assembly draws combustion gases from the fire box 8 through the secondary heat exchanger 17 and the tertiary heat exchanger 19 and discharges the gases through a flue pipe 27. The secondary header depending portion 101 is formed with an inlet 108 on its top surface to allow the inducer fan to draw directly from the secondary header without significantly drawing condensate into the inducer fan. The inducer fan assembly 26 comprises a motor 110 and an impeller 112 with a capacity to draw combustion air and gases through the heat exchanger and exhaust them through the equivalent of 60 feet of two inch diameter piping. As will be described below, the combustion products may be directly vented to the outer atmospheric air through either a co-axial intake/exhaust pipe 120 (FIG. 14) or through a convention chimney or flue. As shown in FIGS. 2 and 3, the inducer fan assembly 26 may also be mounted on a horizontal portion of the secondary header. As shown in FIG. 14B, the inducer fan motor 110 has a corrosive resistant shaft 114, and a corrosive resistant housing and impellor. Because the three-stage heat exchanger of the space heater is very efficient, the exit temperatures of the combustion products are no greater than 125° F. degrees. This allows the inducer motor housing and impeller to be made from low cost materials such as PVC or plastic. As discussed previously, the inducer fan speed may be set for a constant rate or may be varied in accordance with the burner operation.

Referring to FIG. 14, once the combustion products exit the inducer fan assembly 26, they are directed to the intake/exhaust pipe 120. The intake/exhaust pipe may have a coaxial arrangement with an outer tube 112 surrounding an inner tube 114. Inlet air is drawn through the outer tube 112 and into the air inlet portion 15, and the combustion products exit through the inner tube 114 as shown in FIG. 14.

Referring to FIG. 15, the condensate pump assembly 25 comprises a pump 130 located in the condensate container 31. The condensate container 31 accumulates condensate entrained in the combustion gases for humidification purposes and may be made from PVC or other low temperature material. The condensate pump 130 pumps condensate from the condensate container 31 through an outlet 134 into a condensate pan 150 (FIG. 2). As will be described in greater detail below, convection air circulating over the top of the primary fire box runs over the top of the condensate pan 150 (FIG. 2) before it exits from the top of the heater through the room return vent 54 thereby automatically restoring air humidity in the room. The condensate pump assembly 25 comprises of a high level alarm sensor 136 and two actuation sensors 138. The sensors 136,138 have probes which extend from a top plate of the container 31 into an interior of the container. The probes are adjustable in the vertical direction in the container interior to adjust their respective set points. The probes measure electrical conductivity of the condensate and have no moving parts. The probes operate via low voltage control circuit provided in the heater, preferably, 12 VDC. Thus, the pump sensing probes 138 actuate the condensate pump 130 when the condensate level in the container reaches a predetermined level. The high level alarm sensor 136 shuts down the space heater when the condensate level in the sump reaches an excess level, possibly indicating that the pump has failed or the actuation sensors are inoperative. The pressure switch and high condensate level alarm sensor are also incorporated into the system to monitor the combustion process to ensure the combustion exhaust products will not exceed the CO/CO² guidelines, as well as protecting the heat exchanger from building up carbon through the secondary and tertiary heat exchanger.

Referring to FIGS. 2 and 3, a condensate pan is provided adjacent the top of the fire box or primary heat exchanger 8. The condensate pan is sized to accommodate the maximum amount of condensate extracted from the combustion process when the heater is operating between 85 and 92 percent and its rated thermal capacity. The condensate tray 150 is arranged in the path of the convection blower 30 so that air entering the room in which the space heater is located is humidified. The pan 150 may be arranged in a generally flat orientation to provide greater surface area for evaporation and humidification. The condensate pan is filled by a stainless steel or aluminum tube which communicates with the condensate container 31 via the pump outlet 134.

Referring to FIG. 2, the combustion fan 30 is located inside and at the bottom of the heater structure 5 in the electrical compartment 50 to ensure that the cooler room air drawn through a louvered opening 32 at the lower front of the heater is continually moving past the combustion fan and the space heater electronic components thereby extending the reliability and life of the combustion fan motor and other components of the space heater. The outlet of the combustion fan 30 is directed to the plenum behind the fire box, first over the tertiary heat exchanger 19, then over the secondary heat exchanger 17, then up and around the top of the primary heat exchanger 8, over the condensate pan 150, and finally out of the space heater through the return vent 54 and into the room.

In one embodiment of the space heater rated for 28,000 BTUs, the effluent may be vented at temperatures below 150° F. degrees. The space heater has the following general characteristics:

-   -   a 160 CFM convection blower fan.     -   a primary heater exchanger fire box made from thin 304 stainless         steel plate having dimensions of approximately 25 inches wide by         17 inches high by 9½ inches deep.     -   a secondary heat exchanger tube comprises a 2½ outer diameter         304 stainless steel tube approximately 35½″ long with 2 9/16″         outer diameter corrugations spaced at 4 corrugations per inch         along its length. The secondary heat exchanger tube is arranged         to exit the center of the rear wall of the fire box primary heat         exchanger and looped half way back over itself to fit within the         confines of the space heater enclosure thereby creating in         effect two passes in the secondary heat exchanger. The secondary         heat exchanger is arranged to lower the temperature of the         effluent combustion products from a temperature in excess of         800° F. degrees to a temperature of between 400° F. and 450° F.         degrees with the space heater operating burners rated for 28,000         BTUs.     -   a tertiary heat exchanger comprising 5 finned tubes, each         approximately 20½″ long with a ¾″ outer diameter with roughly         1½″ diameter fins spaced at nominally 11 fins per inch along the         length of the tube. The tertiary heat exchanger has been found         capable of reducing combustion gases introduced at a temperature         of between 400° F. and 450° F. to a temperature of no greater         than 150° F.

The space heater of the present invention is very compact and portable and usable in other applications, such as conventional HV AC, gas logs, and hot water heaters. The heat exchangers in the space heater may be appropriately scaled for applications rated for 32,000, 36,000, 64,000 or 120,000 BTUs, while maintaining their compact and efficient arrangement. Gas and fan controls ensure that the desired room or space temperature is maintained. Automatic thermostat controls may be used with the space heater and computer controlled solid state electronic controls may be built into the heater to provide safety and efficiency of the heater in use.

In one embodiment, described in further detail below, the computer controlled solid state electronic controls comprise a microprocessor. State diagrams associated with the microprocessor control are shown in FIGS. 16-18. A general arrangement of the inputs to and outputs from the microprocessor control is shown in FIGS. 19 and 20.

The hardware and firmware of the heater are arranged such that, the microprocessor deals with all functional operations of the space heater. Any fault conditions that are detect, are dealt with by the microprocessor and its firmware on a first line basis. This is achieved, by having the timings shorter for the firmware control, than the hardware supervisory circuits. There are some exceptions to this, in that the flue pressure, external watchdog timer and the external disable signal, cause the gas valves to be de-energized, without processor intervention.

The electronics hardware comprises a power supply, a control microprocessor, input signal conditioning, output drivers and isolation and safety monitoring and cutout functions. These are arranged so that failure of the microprocessor will result in the gas valves being disabled. There are a number of mechanisms to ensure this:

-   -   Failure of the microprocessor to service the internal watchdog         timer within the allotted timeout of approximately one (1)         second will result in the microprocessor being reset. This reset         will lock the program into a loop, locking up all functions.         Thus, the external watchdog will cause the gas valves to be         disabled.     -   Failure of the micro processor to service the external watchdog         timer within the allotted timeout of approximately 1 second.         This will allow the external watchdog circuit to disable the         valves.     -   Failure of the gas valves to be turned off, if no flame detected         within 15 seconds. With the control micro processor working         correctly, the no flame condition would be detected within the         allotted time out. If however this does not occur the external         cutout will take place.     -   Failure of the flue fan to effectively reduce the pressure         within the flue. This directly disables the valves relay         drivers.     -   An externally wired disable input, which may be driven by such         devices, as a gas or smoke detector that once connected to the         ground/common contact, directly disables the valves relay         drivers.

The circuit module will be described below in greater detail. Firmware within the micro processor monitors the inputs and controls the gas valves 33, fan motors 26,30, igniters 66,68, pump 130 and indicators 42,44. To provide the necessary features for both function and safety, the microprocessor is equipped with an internal watchdog timer and an external watchdog timer, the output of which feeds a safety cutout circuit.

The various input signals into the microprocessor are conditioned. For instance, for the flame detectors 70,72, a low power 120 V AC 10 kHz signal is fed to the flame detector rods and earth. A flame from the earthed gas jets, rectifies this signal and so the DC value will be altered slightly. Only the DC value of the signal is considered. The small DC offset is detected and amplified to logic levels. The signal is generated by firmware in the microprocessor. Failure of this signal results in the failure to detect a flame, which will disable the gas valves.

The signal from the sump level detectors 136,138 is also conditioned prior to its processing in the microprocessor. As described above, the passage of air though the heat exchangers causes condensate to form within it which runs down to the sump container. The sump level detectors 138 determine when the pump should be started to reduce the level in the sump. The high level sensor 136 is placed near the top of the sump to determine if the level has risen an excessive amount possibly indicating that the pump 130 may have failed. The signal from the high level sensor effects turning off of the heater, thereby preventing the formation of additional condensate.

The signal indicating the fire box temperature is also conditioned prior to its processing in the microprocessor. The signal is generated via a thermistor 160 (FIGS. 3,4) mounted on the side of the fire box. The fire box temperature signal is used to control the multi-speed convention fan 30 blowing air from the room over the heat exchanger. As the fire box increases in temperature, the speed of the convection fan increases. Increasing convection fan speed will eventually cool the firebox. In one embodiment, the convection fan is operated at a speed setting which generally corresponds to the number of burners operating. Thus, if the front (smaller) burner is operating alone, the fire box temperature control will generally sense a lower temperature and generate a corresponding signal to operate the convection blower at a low setting; if the rear (larger) burner is operating alone, the fire box temperature control will generally sense a medium level of temperature and generate a corresponding signal to operate the convection blower at a medium setting; and if both burners are operating, the fire box temperature control will generally sense a higher level of temperature and generate a corresponding signal to operate the convection blower at its higher speed.

The signal indicating the secondary header temperature is also conditioned prior to its processing in the microprocessor. When the temperature in the secondary header is acceptable, the temperature sensor 104 generates a signal that enables power to the gas valves to keep them open and operating. If the temperature in the secondary header exceeds a set point, the sensor 104 generates a signal that effects disconnecting power to the gas valves, thereby shutting down the space heater.

The sensor 160 sensing fire box temperature also generates a signal that enables power to the gas valves to keep them open and operating. If the temperature in the fire box header exceeds a set point, the sensor 160 generates a signal that effects disconnecting power to the gas valves, thereby shutting down the space heater.

The signal from flue pressure detector 106 is also conditioned prior to its processing in the microprocessor. The sensor 106 comprises both normally on and normally off contacts with the normally on signal being fed to the safety circuits. The flue fan is used to evacuate any unburnt gas and fumes from the heater and environs prior to emission and ignition of the gas from the burner jets. Confirmation of the proper operation of the flue fan is determined by a mechanical diaphragm switch set to toggle at 35 Pa below atmospheric pressure caused by the reduction of the pressure in the flue in comparison with the atmospheric pressure. In one embodiment, the pressure detector is a model RSS 495/498 provided by Cleveland Controls, a division of UniControl, Inc. of Cleveland, Ohio 44109. If the flue fan is working properly, the pressure in the flue should be less than atmospheric pressure and the switch will activate.

As an extra safety feature, the controller is also design with an external disable circuit. This feeds directly to the safety cutout circuit and disables the valves from being turned on. This could be used in conjunction with an external smoke or gas detector. The circuit element involved with this is a diode.

The microprocessor drives certain output devices while being isolated therefrom. Apart from the indicator LEDs on the front panel 42,44, all output devices are driven by 240 V AC, and are switched by semiconductor Triacs or in the case of the gas valves, by relays. The switching signal for the Triacs is derived from an opto-isolated triac driver, which only switches at, or near, the zero crossing portion of the AC power waveform. Some of the AC outputs have detectors to determine if the load, for example the igniters 66,68 are still operational or disconnected. This is also the case for the main valve 33, as it is possible for the over temperature switch to disconnect the power from the valves. This can be detected by the microprocessor, and a fault condition alerts the user to a problem to be rectified.

In one embodiment, isolation is provided by the opto-couplers (M0C3063) which are rated at 7500V for 1 second, 600 volts continuous. Design of the PCB layout ensures the continued validity of this isolation by keeping all high voltage traces and components in an isolated area.

In one embodiment of the space heater, the space is provided with three gas valves: a main gas valve 33, and front and rear gas valves 170,172 (FIGS. 7,8). Each gas valve is switched via a relay. For instance, the relay may comprise a 1 pole 3A Slim Type Relay FTR-F3 Series provided by Fujitsu Components America of Sunnyvale, Calif. One relay is used for each of the burner valves, and another relay having contacts are in series, for the main valve. These relays provide isolation from the 240 V AC power source. The positive input drive of the relay coils are each driven from control outputs of the microprocessor. These turn on the various burner valves in accordance with the desired operation of the heater. The common negative power lead for these relay coils is controlled by the disabling circuits.

In one embodiment of the space heater, a number of conditions will disable the operation of the gas valves, without regard to the microprocessor. Inputs to this circuit are: (i) flue pressure sensor normally closed contact; (ii) the external watch dog timer reset output; (iii) external disable signal; and (iv) flame detector combined with valve drive signal delayed to a greater timeout than the microprocessor programmed “wait for flame” timeout. If the flue pressure drops below the set limit, the gas valves are disabled. This occurs whenever the heater is turned off, so in the static state, the heater is in the shutdown mode. If the external watchdog timer is not strobed within the time limit, the gas valves are disabled. If the external disable signal is shorted to ground, the gas valves are disabled. If the valves are being turned on and no flame is detected, then 15 seconds later the gas valves are disabled. If this is greater than the 10 seconds total, then the microprocessor will wait for the flame to stabilize. If the microprocessor does not turn off the gas valves, the external circuit will do so 5 seconds later. It should be noted, that errors in the micro processor, would most likely fail to strobe the external watchdog, and in so doing, disable the gas valves within 1.2 seconds, well before the flame detectors have registered errors.

The flame detector signal is combined with valve drive and delay circuits. The circuit element used to combine the flame detector output and the valve drive is an exclusive “or” gate. Below illustrates the logic of these signals and the gate. Flame Detector Valve Drive off Output 0 0 0 0 1 1 fault 1 0 1 fault 1 1 0 These input signals are fed to a switching element which disconnects the negative power feed to the relay coils. This action also latches the element in the “off” condition so the valves remain off, even if the disabling signal is removed. The latch is reset by the micro processor, prior to safe operation. This latch is set whenever a disabling signal is active (active=low), thus every time the flue pressure is low, as in the off condition, the gas valves are disabled. Consequently, the microprocessor must re-enable this latch, before further operation can take place.

The space heater may be provided with a front panel circuit that contains the interface to the switches 40,44 and thermostats, as well as the low voltage LEDs that form the indicators 42 illuminating through the front panel. The front panel circuit is connected via ribbon cable to the main PCB.

The microprocessor may be arranged to operate the inducer motor at a variable rate depending upon burner operation. As the microprocessor processes signals to activate one or more burners, the microprocessor is programmed to signal the inducer motor controller to increase the inducer fan speed. For instance, in the burner arrangement shown in FIGS. 7 and 8, the inducer fan may operate at three speeds: a first speed when the smaller rated front burner is operated; a second speed higher than the first speed setting when the rear burner is operated; and a third speed higher than the second speed setting, when both the front and rear burners are operated. The microprocessor may process the signals when the front controls 40,44 are activated or when a thermostat associated with space heater passes through pre-determined set points. As discussed below, when the microprocessor enters the PURGE state discussed below, the inducer fan may be brought up to speed in accordance with the burner to be operated.

In one embodiment, the firmware for the gas fire controller has the following structure: (i) the source code is written in C; (ii) the target processor is MC9S12C32; and (iii) the compiler used is Imagecraft ICC 12 Ver. 6.16A. The source code directory may be structured as follows. Root. −> Project files bin −>compiler output include −> header files of main source main −> main source code files OS drv −> processor specific source and header files

The operational structure of the code may be as follows. initialization of hardware and software. endless super loop Polling COP Polling Main PCB Inputs and Outputs Polling front panel PCB inputs and outputs Polling pump functions Polling convection fan functions Polling finite state machine. Within the endless loop, the independent functions of the controller are maintained: (i) the computer operation properly timer is reset; (ii) the inputs and outputs of the main PCB are read from and written to; (iii) maintaining the various variables for the other functions; (iv) the pump is serviced by checking the level sensor values and determining the need for the pump to operate; (v) the convection fan is operated at various speeds determined by the value of the temperature sensor on the fire box; (vi) the code which determines the state and action of the fire controller is executed. This is the main function of the code, where the flue purge, ignition sequence and shutdown occurs.

One interrupt is used to provide a stable time base for the timers used to sequence various features. All other interrupts lead to the safe shutdown and lockout (halting) of the code.

The microprocessor has several finite states as shown in FIGS. 16, 17 and 18. The microprocessor creates a number of discreet states, and one “super” state for the ON condition, which itself contains a number of discreet states. Initial entry into the state diagram is the reset condition, which then defaults to the OFF condition. A transition out of this OFF condition is via: (i) a user input in the form of the front panel switches turning on either or both burners; (ii) a fault in the flue pressure being detected low: or (iii) a flame being detected.

Each of the states will be discussed below in greater detail. Normal operation will be a transition from the OFF state the FLUE WINDUP state. The FLUE WINDUP state allows a delay before testing the flue pressure to allow the flue fan to get up to speed and evacuate some of the air in the flue. Once the flue pressure has dropped, this triggers a transition to the PURGE state.

The PURGE state occurs during the initial running of the flue fan, when fumes and unburnt gas is evacuated through the flue. This purge lasts 40 seconds, during which time the hot surface igniters are tested for failure. After the 40 second purge the PURGE state moves to the ON super state.

In the ON super state, the first discreet state is the OFF sub-state where the microprocessor switches the valves of the transits to the WARMUP state. The WARMUP state is a 5 second preheat stage for the hot surface igniter. During this time the valves are off. After the 5 seconds, the valves are turned on and the state transits to the IGNITE state. The IGNITE state is the 4 second period within which the gas should be ignited. After this 4 seconds the hot surface igniter are turned off, and the state transits to the HOLDOFF state. The HOLDOFF state is time during which the flame is expected to be detected. If after 5 seconds the flame is not detected the state transits to the UNLOCK state with the gas valves off. If the flame is detected the state transits to the ON state. The ON state is the stable state of the controller when the space heater is operating normally. Within this state, the controller monitors the flame detectors and flue pressure, and turning off the heater, as appropriate. For a two burner system, two sets of states are kept in the ON super state, one for each burner.

The microprocessor may have the following program files and functions: Main.c void main(void) /* Application entry point, initialization calls, main loop. */ This is the start point of the program. The hardware and software variables are initialized, then the main program loop is executed. This is an endless loop, within which the features, both operational and safety are carried out. The static void init(void) function also resides in the main.c file, and is used to call the various initialization functions. Pump.c / * Pump controller. */ This file contains the function to initialize and manage the sump pump and level detectors. The void pump_poll(void) function is called from the main endless loop code in main.c. / * Pump is run when water is detected, and for TIME_PUMP after is * no longer detected; this is to prevent oscillation. * Call this function periodically. */ Fan.c /* Convection Fan Controller. / This file contains the function to run the convection fan. This is determined by the temperature of the fire box, and runs at 3 different speeds, void fan_poll(void) This function is called from the main endless loop code in main.c. / * Fan is run at the appropriate speed when a particular temperature * is reached, and at the lower speed when the temperature is dropped * below. There is a delay when dropping down speeds to prevent * oscillation. There are four speeds (off, low, med, high) based around *three temperature points */ fsm.c */State machine for sequencing functionality. */ This file contains the functions to implement the Finite State Machine used to sequence through the various operational and safety features of the heater.

The main states of the microprocessor are shown below in the following chart: ENTER Start the FSM system. RESET Reset all the states. OFF The stable heater off state. LOCKOUT TEST A transient state that tests the operation of the lockout circuit on the main valve. ON The stable heater on state with either one or both burners. FLUE WINDUP The delay as the fan starts up, before a lack of flue pressure will cause a fault. FLUE_WINDDOWN The delay as the fan slows down, before a false flue pressure will cause a fault. PURGE The start of the ignition sequence, evacuation of the fire box and flue. FAULT_NO_FLUE_PRESSURE A transient state which shuts down and sets the front panel LEDs, then moves onto the Fault Recovery state. FAULT_FALSE_FLUE_PRESSURE A transient state which shuts down and sets the front panel LEDs, then moves onto the Fault Recovery state. FAULT_HIGH_WATER A transient state which shuts down and sets the front panel LEDs, then moves onto the Fault Recovery state. FAULT_FLAME_OUT A transient state which shuts down and sets the front panel LEDs, then moves onto the Fault Recovery state. FAULT_RECOVERY A state that determines the shutdown path, either restarting after a turnout, or going into a lockout, which requires the front panel switches to be both turned off. LOCKOUT A static state which shuts done the gas flow and requires the front panel switches to be both turned off.

The discreet states of the microprocessor within the ON “super” state are shown in the below chart. ENTER A transient state which moves onto the ON superstate OFF state. OFF A transient state which makes sure the burner valve is off, and if the specific burner is required, moves onto the hot surface igniter warm-up state. WARMUP A static state which times the warm-up phase of the hot surface igniter's operation IGNITE A static state which times the ignition phase of the hot surface igniter's operation HOLDOFF The state which times the flame stabilization phase of the ignition sequence. UNLOCK A transient state that is the default failure mode of the ON superstate, this moves to retry the ignition via the off state. ON The stable state of the normal operation of the burners. Thermostat or front panel controls will move the state to the OFF state within normal operation. Any failure will change the state to the appropriate shutdown or lockout state. There is a function for each of the states that is called with a passed variable of the type, transition_t, which can be: (i) transition_enter, which sets the function variables to support the new state; (ii) transition_do, which performs the operations required of that state; or (iii) transition_exit, which sets the variable and performs the operations to exit this state and move to the next state. The “call to” the appropriate state function is done by a call to “fsm_do( )” within the endless loop in the main.c code. The call to fsm_do( ) uses a function pointer lookup table to call the state function.

The function “m_function_lookup a[m_state] (Transition_do)” is called with the “Transition_do” variable. With this passed variable, the functions of that state are performed, including determining if a transition is required. If the transition to another state is required, that state's function is called with the “Transition enter” variable, which sets the state the new state. So the next call will be to this function with the “Transition_do” variable and the transition to the next state is invoked by the macro TO_STATE. For example: TO_STATE(State_FAULT_FALSE_FLUE_PRESSURE, fp_flue(FpFlue_switch_fault); #define TO_STATE (new_state, transition_code) \ m_function_lookup_a[m_state] (Transition_exit); \ {transitioncode} \ m_state = new state; \ m_function-lookup_a[m_state](Transition enter);

-   At the point that this call is invoked, the “m_state” is still the     present state, and so the call m_function_lookup a[m_state]     (Transition_exit); calls the same routine to perform the exit state     operations. This then depends whether the functions used in this way     are re-entrant. Any required function calls are then made by the     line:

{transitioncode} \

The m_state is then changed to the new state, and the function for that code is called by the line:

m_function_lookup_a[m_state] (Transition_enter);

For transition within the “ON” super state, the above program flow is slightly modified as follows: #define TO_STATE_ON(burner, new_state, transition_code) \ m_ON_function_lookup_a[m_ON_state_a[bumer]](burner, Transition_exit); \ {transitioncode} \ m_ON_state_a[burner] = new_state; \ m_ON_function_lookup_a[m_ON_state_a[bumer]](burner, Transition_enter); As there are 2 burners that have an ON state, the burner in question is added to the Macro.

The microprocessor may be provided with fault sensing capabilities and diagnostics. Depending upon the nature of the fault, the system will may: a) become inoperative with all valve terminals de-energized; b) proceed to safety-shut-down, or lockout; c) continue to operate, the fault being identified at the next startup sequence; and/or (d) remain operational. FIG. 21 shows a chart of the fault responses processed.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.

As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents. 

1. A space heater comprising: a burner assembly having a plate supporting a burner and burner valve operatively connected to a gas supply regulated by a main gas valve of the space heater, the plate supporting an ignitor for igniting gas discharged from the burner, the plate supporting at least one ignition sensor configured for sensing the presence ignited gas exiting the burner; a fire box having a generally box shape with left and right heat exchanger surfaces, top and bottom heat exchanger surfaces, and a rear heat exchanger surface defining an interior of the fire box, the burner assembly disposed in the fire box interior, the fire box having a fire box thermistor adapted to sense fire box temperature; panels spaced from and surrounding portions of the fire box to form a plenum; a secondary heat exchanger disposed in the plenum and exterior to the fire box adjacent the rear heat exchanger surface of the fire box, the secondary heat exchanger operatively connected to the fire box and receiving a flow of combustion gases from the fire box, the secondary heat exchanger comprising a corrugated flexible steel tube with a turbulator disposed inside the tube to induce spiral flow of the combustion gases passing through the secondary heat exchanger; a primary header disposed in the plenum and exterior to the fire box adjacent the rear heat exchanger surface of the fire box in a generally vertical arrangement, the primary header operatively connected to an outlet of the secondary heat exchanger; a secondary header disposed in the plenum and exterior to the fire box adjacent the rear heat exchanger surface of the fire box in a generally vertical arrangement, the secondary header having a temperature sensor adapted for sensing temperature of combustion gases flowing through the secondary header; a tertiary heat exchanger disposed in the plenum and exterior to the fire box adjacent the rear heat exchanger surface of the fire box below the secondary heat exchanger, the tertiary heat exchanger comprising a plurality of finned tubes supported by and communicating with the primary header and the secondary header whereby combustion gases exiting the secondary heat exchanger pass through the primary header into the tertiary heat exchanger and flow into the secondary header, the tubes of the tertiary heat exchanger having turbulators disposed therein for inducing a spiral flow therein; a condensate collection assembly arranged in the plenum below and operatively connected to the secondary header through a depending flow portion of the secondary header, the condensate collection assembly comprising a pump and a sump, the pump being configured to pump condensate entrained in the combustion gases from the sump to a condensate tray disposed atop the top heat exchanger surface of the fire box, the condensate collection assembly having at least one sensor adapted to sense a level of condensate in the sump; an inducer mounted on the depending flow portion of the secondary header, the inducer being configured to draw combustion gases out of fire box and through the secondary and tertiary heat exchangers and discharge to a flue having a pressure sensor for monitoring pressure in the flue downstream of the inducer; a convection fan located in the plenum configured to draw air from a room in which the space heater is situated into the plenum below the fire box and to pass the air over the secondary and tertiary heat exchangers and over the condensate tray before discharging the air to the room; and a microprocessor control for the space heater being configured to receive signals from at least the (i) ignition sensor associated with the burner, (ii) the flue pressure sensor, (iii) the secondary header temperature sensor, (iv) the fire box thermistor, and (v) the condensate sump level sensor, and in response thereto, control at least the operation of (a) the inducer, (b) the convection fan, (c) the condensate pump, (d) the burner ignitor, (e) the burner gas valve, and (f) the main gas valve.
 2. The space heater of claim 1 wherein the secondary heat exchanger tubulator is made from a material that is anodic relative to the combustion gases and the material forming the tube of the second heat exchanger.
 3. The space heater of claim 1 wherein the tertiary heat exchanger tubulators are made from a material that is anodic relative to the combustion gases and the material forming the finned tubes of the tertiary heat exchanger.
 4. The space heater of claim 1 wherein an interior of the tubes of the tertiary heat exchanger are accessible via at least on of the primary and secondary header.
 5. The space heater of claim 1 wherein an interior of the tube of the secondary heat exchanger is accessible via the primary header.
 6. The space heater of claim 1 wherein the burner assembly includes first and second burners.
 7. The space heater of claim 6 wherein the microprocessor operates the inducer at a variable rate depending upon a number of burners in operation.
 8. The space heater of claim 6 wherein the microprocessor is further configured to sense a temperature in the room and operate at least one of the first and second burners in response thereto.
 9. The space heater of claim 8 further comprising a thermostat configured to generate a signal in response to a temperature in the room.
 10. The space heater of claim 6 wherein the first and second burners have different heat generation ratings.
 11. The space heater of claim 10 wherein the burner plate has openings adjacent each burner and the openings are dimension to regulate air flow to the burner to maximize combustion in accordance with the burner rating and inducer fan capacity.
 12. The space heater of claim 1 wherein the flue comprises an inner pipe surrounded by an outer pipe and air for combustion flows through the outer pipe before being introduced to the burner.
 13. The space heater of claim 12, wherein the inner and outer pipe are made from a polyvinyl chloride material.
 14. The space heater of claim 1, wherein the tertiary heat exchanger tubulators are removably insertable into the finned tubes of the tertiary heat exchanger.
 15. The space heater of claim 1 wherein the secondary heat exchanger turbulator is removably insertable into the corrugated tube of the secondary heat exchanger.
 16. The space heater of claim 1, wherein the secondary header is made from a polyvinyl chloride material. 